This invention relates to a semiconductor device and process for producing the same, and particularly to a process for producing a semiconductor device suitable for forming a thermal oxide film and a semiconductor device obtained by said process.
In the production of semiconductor elements using silicon as a substrate, silicon oxide film formed by thermal oxidation of silicon is used as an insulating film. In the process of forming this thermal oxide film, a silicon-oxygen bonding is formed while cleaving silicon-silicon bonding, due to which a great strain (stress) appears in the vicinity of interface between silicon and oxide film.
Since molecular volume of silicon oxide is twice or more as great as that of silicon, the oxide film formed by oxidation reaction tends to expand, due to which a tensile stress appears in the silicon side and a compressive stress arises in the oxide film side, usually. When stresses become great, crystal defects such as dislocation and the like appear in the silicon substrate which is a single crystal. In semiconductor elements, the presence of such crystal defects causes leakage current and greatly deteriorates reliability of article.
Even if no crystal defect appears in silicon substrate, the stress arising in the oxide film can strain the atomic distance in the oxide film and thereby lower the atomic bonding forces and, in some extreme cases, cause injuries such as breakage of atomic bondings. If such an injury appears, insulating characteristics of oxide film are deteriorated, and electrical reliability of oxide film and article decreases.
Generally speaking, the value of stress monotonously increases as thickness of the formed oxide film increases. Accordingly, when a thick thermal oxide film is to be formed, relaxation of the stress is an important problem. As a method for relaxing the stress, JP-A-3-11733 proposed a method of once interrupting the thermal oxidation, carrying out a heat treatment to eliminate strains, and thereafter again continuing the thermal oxidation.
From the viewpoint of mechanism for generating stress in oxide film, the stresses can be classified into a stress caused by the volume expansion of oxide film in the vicinity of silicon/oxide film interface brought about by the oxidation reaction in the oxide film-forming process and a stress generated from the thin films deposited on oxide film.
Although the stress caused by oxidation reaction can be relaxed to some extent according to prior technique, there has hitherto been no effective method for relaxing the stress generated from the thin films deposited on oxide film. Said "stress generated from the thin films deposited on oxide film" is generated according to the following process.
First, as the process of formation of oxide film, there can be referred to a process of partially forming the elements-separating oxide film up to a thickness of about several thousands angstroms for the purpose of electrically insulating and isolating the elements, such as transistors, adjacently placed on a silicon substrate. As a method for forming such an element-separating oxide film, the selective oxidation method is widely utilized (cf. FIG. 2). According to the selective oxidation method, a silicon nitride film 3 (FIG. 2C) is deposited on a silicon substrate 1 (FIG. 2A) through intermediation of a thin thermal oxide film called "pad oxide film 2" (FIG. 2B), and then the silicon nitride film 3 is etched off from the region on which an element-separating oxide film is to be formed (FIG. 2D) and the whole is oxidized to form a thick oxide film partially on the silicon substrate (FIG. 2E).
In this selective oxidation method, the silicon nitride film used as an oxidation-preventing film has an internal stress of about 1,000 MPa at the time of depositing the films, in many cases, and this stress acts upon the oxide film, too. Further, in the process of selective oxidation, oxidizing species such as oxygen and H.sub.2 O three-dimensionally diffuse in the silicon substrate, as a result of which oxide film 5 called "bird's beak" grows in the vicinity of the edge of silicon nitride film.
Since volume of an oxide film expands in the growing period of the oxide film, edge of the silicon nitride film is lifted, and a warping deformation appears in the whole film. Reaction forces cause by this warping deformation are concentrated into the edges of silicon nitride, and a great stress appears in the oxide film at the edges of silicon nitride film. This concentration of stress takes place without fail to injure the oxide film when a silicon nitride film exists.
Another process through which the thin film deposited on oxide film injures the oxide film is the process of depositing a thin film as a gate electrode on the gate oxide film of MOS (metal oxide semiconductor) type transistor. As the gate electrode, polycrystalline silicon thin film, high-melting metallic material or silicide alloy thin film is used either in the form of single layer or in a laminate structure.
Such a gate electrode material is often deposited with an internal stress exceeding several hundreds or one thousand MPa. Thus, when a gate electrode is fabricated, the internal stress is concentrated into the oxide film in the vicinity of edge parts of gate electrode, and thereby the oxide film is injured.